A procedure which currently is employed for determining the presence of an explosive gas involves passing a beam of infrared radiation through a sample chamber and detecting the level of absorption of the radiation within the chamber. An output signal is derived which provides a measure of the infrared radiation which impinges on a detector, with an increase in the level of the output signal indicating a reduction in the level of infrared absorbing gas in the chamber. A filter is normally located between the radiation source and the detector to selectively pass an infrared frequency which, by its absorption, is indicative of the presence of a gas of interest.
In order to accommodate the existence of background radiation, which normally increases with increasing ambient temperature, the beam is modulated and an adjustment is made to compensate for the level of background radiation. In one known prior art system this is achieved by switching an infrared source on and off, measuring the level of radiation which is incident on the detector during an ON period, measuring the level of incident radiation during an OFF period and subtracting a signal which is representative of the second level measurement from that which is representative of the first level measurement. Any steady background infrared radiation will make the same contribution to both signals and will be cancelled when the two signals are subtracted.
A problem which is known to exist in the above described prior art system is that false readings are obtained when the background radiation level varies with time. If the background radiation changes between the OFF period and the ON period measurements of incident radiation, a false reading will be obtained when the signals that are representative of the two levels are subtracted one from the other. Given that gas detection may be required in hazardous environments, such as on oil rigs and in petrochemical plants, it is important that fast responses be obtained to indicate accurately the existence of any explosive gas mixtures and that false indications be minimized.